The ability of the body to control the flow of blood is paramount to healthy cardiovascular functions. The process of blood clotting and the subsequent dissolution of the clot, following repair of injured tissue, is termed hemostasis and comprises the clotting cascade. Several coagulation factors are involved in the cascade that results in the activation of thrombin and ultimately in the formation the fibrin clot.
Among these factors, the pivotal function of the blood coagulation proteins, Factor Va (FVa) and Factor Xa (FXa), is to activate prothrombin (II) to thrombin (IIa), which is directly responsible for producing fibrin clot (FIG. 1).
As the inactive precursor of coagulation FVa, FV is an essential clotting protein. FVa functions to accelerate the factor Xa-dependent production of thrombin by five orders of magnitude. The generation of both FVa and FVa activity is strictly regulated by physiological anticoagulants. Too much activity increases the risk of thrombotic disease, whereas too little results in severe hemophilia. An example of the former is the most prevalent inherited coagulation mutation (arginine 506 to glutamine) resulting in a form of Va, termed Va Leiden, that is protected from the key anticoagulant, activated protein C. Transfusion with normal plasma to ameliorate the effect of V Leiden leads to even more clotting activity. Therefore prevention of further thrombosis in these patients is controlled indirectly by use of therapeutics that lead to reduction in the activity of other clotting proteins.
Part of the molecular mechanism by which FV and FVa ultimately activate prothrombin (II) and thrombin (IIa) is understood. FVa and factor Xa must simultaneously associate with anionic phospholipid in the presence of Ca 2+ to form the physiologically relevant enzyme-cofactor complex, prothrombinase. Using plasma-derived human FVa, we recently reported (Zeibdawi, A. R. et al. (2001) J. Biol. Chem. 276 (23), 19929-19936) that leucine 94-lysine 109 (L94-K109) within the A1 domain appears to play a critical part in the FVa light chain (FVaL) and FVa heavy chain (FVaH) Ca2+-dependent association. However, other structure-function relationships for the L94-K109 region remain to be elucidated to more fully understand their role in thrombin formation. Furthermore, it will be appreciated that further understanding of this structure-function relationship is needed to devise new therapeutic strategies for the various known coagulation diseases.